Spirit of 74 (oz13785)
About this Plan
Spirit of 74. Radio control sport model.
Quote: "A small-field, fun airplane that provides a home for the new Queen Bee. Spirit of 74, by Randy Randolph.
The new Cox .074 is a good engine. At this stage of its history, it's not a great engine, like the TD .049, but the potential is there. It fills the gap between the .049s and the .099s for the first time since the OK Cub and Fox .07 engines of the '50s. but, unlike the Cub, this engine is a horse! The .074 should find a place in smaller R/C airplanes for some time to come.
The Spirit of 74 was designed to take advantage of the additional power available from the .074. It's a little larger than the typical .049 designs and, even though it's a 3-channel machine, the additional power and the tip dihedral result in a very smooth-flying airplane. The steerable tail wheel was added to take advantage of the throttle so the Spirit could be taxied out just like the 1/4 scale birds. There's plenty of room in the fuselage for full-size radio equipment and enough wing to handle the additional load. The shoulder-wing configuration was selected to facilitate hand-launches from unimproved fields.
The G-Mark .061 would be a good substitute for the .074. To utilize the G-mark, the fire wall should be moved forward so the engine drive washer is in the same plane as that shown on the plans for the .074. The only other change would be to exchange sides with the throttle and fuel lines. The throttle response of the G-Mark is somewhat better than that of the .074, and it's a good trade-off for the loss of a little power.
Construction: The wings are a good place to start, since they take a little more time. But once the pieces are all laid out. they go together easily. You'll need 23 ribs cut from 1/16 sheet balsa. They can be cut from a printed sheet made by tracing around a card-stock template with a pen, or they can all be cut at the same time by stacking balsa blanks together, tracing the rib pattern on top and sawing them with a band saw or a jigsaw. If you use the printed-sheet method, after they have been cut from the sheet, they should be stacked and pinned together, then gang-sanded to smooth out any high or low places that might have crept in during the slicing.
For the center-section ribs, select three ribs and trim 1/16 inch from the top and bottom of each. Cut the webs from 1/16 sheet. Webs add a lot to the strength of the spars and add little weight. The spars are stripped from straight-grained 3/16 sheet. This can be done by using a straightedge razor knife, or one of balsa strippers on the market. Slice the tip pieces from soft 1/8 sheet, and notice the direction of the grain in each piece.
The wing is built in three sections. Cover the plan with wax paper, and start building the center section by pinning the bottom main spar into place on the plan. Slip some ribs onto the spar. and use them to position the trailing-edge sheet so that it will match any slight difference there might be in the length of your ribs and the plan. Pin the trailing edge into place, and, starting with the three trimmed center ribs, glue ribs and spar webs into position. When installing the end ribs, use the template to trim the web to the proper angle so the ribs will be slanted to make the dihedral.
When all ribs and webs are installed. add the top main spar. Make sure that it's glued to all the webs as well as to the ribs. The leading edge and from top spar can be installed at this time, but don't add the top trailing-edge sheet yet. It will be installed after the wings have been joined at the dihedral joints..."
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